Optical storage media typically takes the form of a thin (approximately 2.5 mm thick), rotatable disk, such as 90 or 130 mm in diameter, having one or two active surfaces to and from which information can be written and read when the disk is mounted in an optical drive. To reduce the accumulation of dust, body oils (from handling) and other contaminants which can interfere with the reliability of an optical system, optical disks have been housed within rigid cartridges, approximately 11 mm thick. A shutter door on the cartridge slides open when the cartridge is mounted in a drive and provides access to a radial portion of the active surfaces of the disk; the shutter then closes when the cartridge is removed.
Such a cartridge can also include a write-protect mechanism whose position is sensed by the drive: when the mechanism is in one position, the drive is prevented from writing information to the disk. Another feature found on cartridges are sensing holes which indicate to the drive, by their coded positions along the rear edge of a cartridge, the type of disk (such as write-once (WORM) or rewritable magneto-optical (MO), single- or double-sided, and capacity).
Because optical cartridges are frequently used in automated storage and retrieval library systems, the cartridges can also include a notch near the rear of the two sides of the cartridge to enable a robot gripper to securely grasp and transport the cartridge between a storage cell and an optical drive.
It will be appreciated that the need for media protection is reduced in a closed library with reduced human handling and a more controlled environment. It will also be appreciated that the number of optical cartridges which can be stored within a library of a given volume is directly related to the thickness of the cartridges.